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Swab tests and judgment calls: How police are checking for pot DUIs

A road sign and cones lead westbound drivers on Alondra Boulevard through a DUI checkpoint manned by Los Angeles County Sheriff's Department deputies March 6, 2014 in Bellflower.
Stuart Palley/KPCC
A road sign and cones lead westbound drivers on Alondra Boulevard through a DUI checkpoint manned by Los Angeles County Sheriff's Department deputies March 6, 2014 in Bellflower.

Recreational marijuana went on sale in California nearly a month ago but questions remain about the best way to enforce the law to keep dangerously stoned drivers off the road.

As of Jan. 1, state law prohibits drivers from having an open container of cannabis inside the car or using the substance while driving. But the rules for use before someone gets behind the wheel are fuzzy.

Ask Nick Zoppo, a frequent pot smoker who lined up last weekend in front of the Med Men pot shop in West Hollywood. Quizzed about driving while on marijuana, he pauses.

"That's a good question," said Zoppo. He said he's been using the drug for about seven years, multiple times a day, to treat anxiety.

"I drive under the influence of marijuana often and, if anything, I feel like it makes me a better driver," he said.

Under California law, it is illegal to drive under the influence of pot. Research proves it can affect depth perception and sustained attention and could double the risk of getting involved in a crash.

But it's possible Zoppo wouldn't be breaking any laws when he drives because it's not illegal to drive after smoking pot unless it's impairing his ability to operate a car.

Measuring impairment from marijuana, though, can be difficult. Alcohol, is water soluble and appears in blood and body fluids in proportionately higher amounts the more one drinks, correlating with a higher level of impairment. On the other hand, the active ingredient THC in marijuana is fat soluble and metabolizes very differently, making it harder to determine driving impairment.

"There is kind of a disconnect between the blood levels and how impaired you really are," said Dr. Igor Grant, director of UC San Diego's Center for Medical Cannabis Research, which is studying the effects of pot on driving.

When someone smokes marijuana, Grant said, the level of THC in the blood will rise quickly within a minute and then dissipate quickly to nearly imperceptible levels as the ingredient is distributed to tissues and organs, like the brain, where it can alter mental processes. The effects can vary widely.

"You could have a high level if you tested somebody immediately and they wouldn’t be very impaired, or you could have a low level, and you could be impaired," he said.

THC levels and impairment are highly individual based on variables like body composition and tolerance level. So an experienced user like Zoppo might very well exhibit no impairment at THC levels that would put another driver to sleep.

That’s why California, unlike Washington and Colorado, has not set a legal limit for THC. But that doesn’t mean law enforcement agencies aren’t worried about the risks.

At a training session for the Los Angeles County Sheriff's Department, Detective Aaron Percy shared concerns about increased traffic safety risks from the broader access to marijuana.

"My family is in danger, your family is in danger," he said. "It increases the danger of those on the road."

Percy teaches a class in Advanced Roadside Impaired Driving Enforcement, a statewide program administered by the California Highway Patrol that prepares law enforcement officials to catch signs of drug impairment when they pull over a driver.

First, deputies look for an odor in the car, red or watery eyes and bumps on the tongue caused by smoking. If the clues match up, they might give a driver a field sobriety test to look for indications of impairment.

Officers could ask a driver to walk in a straight line or touch his or her nose. But what they look for is different than for those who are drunk. A stoned person may be able to walk straight, but they might not be able to cross their eyes, a symptom called lack of convergence. They also may have an elevated heart rate and blood pressure and difficulty following complex instructions.

If a driver is deemed to be impaired, he or she will be arrested and subjected to a blood toxicology test at the police station. That’s a lot of time and work by officers before getting proof. And because of the way the drug is metabolized, THC is present in increasingly smaller amounts after use that make it impossible to determine when the drug caused impairment.

There’s now a rush to develop a tool — like an alcohol breathalyzer — that can test for THC immediately at the roadside when drivers are stopped.

The Los Angeles Police Department has been experimenting with a device that can detect THC and six other drugs in saliva with an absorbent swab.

Officer Kamaron Sardar of LAPD's Drug Recognition Expert unit demonstrated the device for KPCC:


LAPD only has four of these machines, so they’re not in every squad car. But the agency is hoping to put them into wider use in coming months at DUI checkpoints or in jails, distributing swabs to officers to take samples that could be analyzed later.

Several companies are working to develop an even more portable device to detect THC in breath where it's present for a shorter time. It would give instant feedback that cannabis was used recently, when its effects are more likely to impair an individual.

But Sardar said all toxicology tests suffer from the same flaw – there is no amount of THC or window of time since marijuana use that can prove that a driver is actually impaired. This uncertainty means the observations of the arresting officers are still the most important tool for law enforcement.

"I’d say that’s still going to be the priority — having enough well-trained officers to go to court to establish and substantiate that impairment that was seen," said Sardar.

LAPD is using the results of the saliva test only to back up impairment evaluations conducted by officers. In fact, the test is designed not to reveal which specific drug is detected so as not to bias the officer's assessment.

"I think where we're at now with cannabis is where they were at in the 60s and 70s with alcohol," he said. "The laws are being passed, kind of putting the cart before the horse, before the science is really there to understand it and we are now faced with technology trying to catch up."

He said there is still a lot of research yet to be done to find better ways to enforce the law. A good chunk of the expected tax revenues from pot sales will do just that.

The California Highway Patrol, local law enforcement and universities like University of California, San Diego will be allocated more than $15 million this year to conduct research. Among the questions they will pusue is whether there could be compounds besides THC that are better indicators for impairment, or whether a cognitive test on a smartphone could yield better results than the standard sobriety test.

But until those questions are answered, keeping stoned drivers off the roads will largely be up to the judgment of police officers and, of course, users like Nick Zoppo.

"I would hope that everyone would know their limit and know when they’re safe to drive," said Zoppo. His advice: "Make sure you can turn and pick up all your limbs, see straight and self-gauge before you get behind the wheel."

Do you have a question about driving while under the influence of marijuana? Tell us below.